1. Field of the Invention
The present invention relates to an x-ray tube for generating x-rays, and more particularly to a right angle type x-ray tube in which a cylindrical electron gun assembly and a cylindrical target assembly are arranged at right angles.
2. Description of the Related Art
X-rays are a type of electromagnetic radiation that easily penetrates many kinds of materials and objects. This phenomenon is used to produce x-ray images to observe insides of various objects without contacting or destructing the objects.
X-ray tubes for generating x-rays have been proposed as one type of electron tube. The x-ray tube is constructed from a cylindrical electron gun assembly and a cylindrical target assembly. In the electron gun assembly, electrons are generated and then accelerated. Electrons are then introduced into the target assembly. Electrons collide with an x-ray target, whereupon x-rays are emitted from the target. There have been proposed two types of x-ray tubes: an axial type x-ray tube; and a right angle type x-ray tube. In the axial type, the electron gun assembly is arranged axially with the target assembly. In the right angle type, the electron gun assembly is arranged at right angles with and intersecting the target assembly.
U.S. Pat. No. 5,077,771 has proposed one example of the right angle type x-ray tube as shown in FIG. 1. This x-ray tube 900 mainly includes an electron gun assembly 910 and a target assembly 920 which are arranged at right angles.
The electron gun assembly 910 includes a heater 911, a cathode 912, a focus grid 913, and an electron gun container 914 for enclosing all these components. The heater 911 generates heat when supplied with electric power. The cathode 912 emits electrons (thermoelectrons) when heated by the heater 911. The focus grid 913 is supplied with electric voltage to accelerate electrons emitted from the cathode 912 while converging the electrons to produce an electron beam. The electron gun container 914 is formed with an opening through which the electron beam passes. The heater 911, the cathode 912, the focus grid 913, and the opening are arranged along an axis of the electron gun assembly 910.
The target assembly 920 includes a target housing 923, interior of which is communicated with the interior of the electron gun assembly 910 via the opening of the electron gun container 914. A hood electrode 922 is provided in the interior of the housing 923. The hood electrode 922 is of a tube shape in which is enclosed an x-ray target 921. The tube-shaped hood electrode 922 is placed with its axis intersecting at right angles with the axis of the electron gun assembly 910. A higher positive voltage is applied to both the hood electrode 922 and the target 921, than to the focus grid 913. Accordingly, the electron beam emitted from the opening of the electron gun assembly 910 is further accelerated toward the target 921 in the hood electrode 922.
The electron beam enters the hood electrode 922, through an electron beam opening formed in the hood electrode 922, to collide with the upper surface of the target 921, whereupon x-rays are generated. An x-ray opening is formed at an end of the hood electrode 922. An x-ray window 924 made of x-ray translucent material is formed on a radiation-emitting end of the housing 923. X-rays generated at the target 921 therefore pass through the x-ray opening and the x-ray window 924 to emit from the x-ray tube.
In the target assembly 920, an electric field is produced due to the potential difference between the focus grid 913 and the hood electrode 922. Electrons move along the electric field while being accelerated toward the hood electrode 922. In other words, electrons move normal to the equipotential surfaces formed between the focus grid 913 and the hood electrode 922. High speed electrons therefore collide with the target 921, whereupon x-rays are produced. The position on the surface of the target 921, at which the electron beams collide with the target, will be referred to as an x-ray generating point P hereinafter.